Current biasing circuit

ABSTRACT

A current mirror circuit is disclosed including a reference device and a biased device, each having control, input and output elements, with the control element of the biased device operably connected to the control element of the reference device. A reference current source is connected to the input element of the reference device and produces a reference current flowing through the reference device, wherein a bias current is produced in the biased device as a multiple of the reference current. A compensation network is connected between the biased device and the reference device for maintaining a constant bias current in the biased device regardless of varying operating characteristics in at least one of the biased device and the reference device.

FIELD OF THE INVENTION

The present invention is directed toward a current biasing circuit and, more particularly, a current biasing circuit compensating for changes in device parameters.

BACKGROUND OF THE INVENTION

Current biasing circuits, or “current mirror” circuits, are generally well known. Current mirrors generally use transistors, FETs (Field-Effect Transistors) or BJTs (Bipolar Junction Transistors), to produce a controlled current in a “biased” device as a multiple of a reference current that flows in a “reference” device. In an ideal case, the multiplying factor depends only upon the geometrical properties of the reference and the biased device.

Current mirrors constructed of conventional transistor devices should come close to the ideal case, where the physical geometry of the transistors is the sole factor influencing errors. Transistors typically used in current mirror devices include MOSFET (Metal Oxide Semiconductor Field-Effect Transistor), MESFET (Metal Semiconductor Field-Effect Transistor), HEMT (High-Electron-Mobility Transistor) and PHEMT (Pseudomorphic High-Electron-Mobility Transistor) devices. The operation of these transistors is based upon the strength of an electrical field in a “channel” underneath a “gate” region. In the ideal case, inaccuracies in the current multiplication factor should relate back only to lithographic errors, which are unavoidable in semiconductor device manufacturing. However, the lithographic errors can be minimized.

If the electrical device transfer functions are ideal, in the sense that differences in the electrical environment or temperature of the reference device and the biased device do not influence the current multiplication factor, then the geometrical errors of the devices define the accuracy limit that can be achieved. However, this is not generally the case, particularly in advanced transistor devices with very short channel lengths; the channel length being the physical length of the gate contact. Various operational parameters influence the current multiplication factor in traditional current mirror devices.

For instance, short channel effects, which result from channel length modulation due to changes in the transistor's drain-source voltage, effect the current multiplication ratio. Velocity saturation effects, which depend on the transistor's drain-source voltage and result from the limited drift velocity of charge carriers in the channel region of the transistor substrate, also effect the current multiplication ratio. Threshold voltage modulation effects also influence the current multiplication ratio. The threshold voltage modulation effects generally result from either a barrier lowering effect caused by increasing drain-source voltage in short channel length transistors, or a barrier increasing effect, particular to short channel length silicon MOSFET transistors, caused by increasing source-bulk voltage. Still further, drain-gate reverse leakage current, common to FETs, has an effect on the current multiplication ratio. The drain-gate leakage current typically results from reverse leakage, including tunnelling, in the gate-source Schottky contact in MESFET devices, or tunnelling through the gate oxide region in MOSFET devices.

The present invention is directed toward overcoming one or more of the above-mentioned problems.

SUMMARY OF THE INVENTION

A current mirror circuit is disclosed including a reference device and a biased device, each having control, input and output elements, with the control element of the biased device operably connected to the control element of the reference device. A reference current source is connected to the input element of the reference device and produces a reference current flowing through the reference device, wherein a bias current is produced in the biased device as a multiple of the reference current. A compensation network is connected between the biased device and the reference device for maintaining a constant bias current in the biased device regardless of varying operating characteristics in at least one of the biased device and the reference device.

In one form, the reference and biased devices include field effect transistors having gate, drain and source elements corresponding to the control, input and output elements.

In another form, the reference and biased currents flow from the drain to source elements in the reference and biased transistors, respectively. The varying operating characteristics include a varying voltage across the drain and source elements of at least one of the biased transistor and the reference transistor.

The varying voltage across the drain and source elements of at least one of the biased transistor and the reference transistor results from at least one of threshold voltage modulation, short channel effects and gate leakage current occurring in at least one of the biased transistor and the reference transistor.

In another form, the compensation network includes a first resistor connected between the input element of the reference device and the control element of the biased device, and a second resistor connected between the input element of the biased device and the control element of the reference device.

The current mirror circuit may further include third and fourth resistors serially connected between the control elements of the reference device and the biased device. A feedback loop is provided between a node common to the third and fourth resistors and the input element of the reference device. Depending upon the types of transistors implemented in the current mirror circuit, the feedback loop may include a unity gain amplifier or a level shifter biasing the reference device to operate in a saturation mode. The first and second resistors may have equal resistance values, and the third and fourth resistors may have equal resistance values.

In another form, the compensation network further includes a compensation device having control, input and output elements, with the input element of the compensation device connected to the input element of the biased device, and the control element of the compensation device connected to the control element of the reference device. A fifth resistor is connected between the output element of the compensation device and ground.

The compensation device may include a field effect transistor having gate, drain and source elements corresponding to the control, input and output elements.

In another form, the compensation network further includes a sixth resistor connecting the second and third resistors to the control element of the reference device, and a seventh resistor connecting the first and fourth resistors to the control element of the biased device.

An object of the present invention is to cancel the effects of threshold voltage modulation in a current mirror device.

A further object of the present invention is to cancel the influence of short channel effects in a current mirror device.

A further object of the present invention is to cancel the influence of gate leakage current related effects in a current mirror device.

A further object of the present invention is to maintain a constant output current in a current mirror device regardless of voltage changes in either the reference or biased device.

Other aspects, objects and advantages of the present invention can be obtained from a study of the application, the drawings, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a prior art current mirror circuit in silicon MOSFET technology using enhancement mode n-channel transistors;

FIG. 2 shows a prior art current mirror circuit in GaAs MESFET technology using depletion mode n-channel transistors;

FIG. 3 shows a prior art biasing circuit utilized in common source amplifiers;

FIG. 4 is a graph illustrating the relationship between drain current and drain-source voltage in a biased transistor of a current mirror circuit due to short channel and threshold voltage modulation effects;

FIG. 5 is a graph illustrating the relationship between drain current and drain-source voltage in a biased transistor of a current mirror circuit due to gate leakage current;

FIG. 6 shows a biasing circuit for a current mirror according to a first embodiment of the present invention compensating for short channel and threshold voltage modulation effects;

FIG. 7 is a graph illustrating the relationship between drain current and drain-source voltage in the biased transistor of the biasing circuit for a current mirror shown in FIG. 6;

FIG. 8 shows a biasing circuit for a current mirror according to a second embodiment of the present invention additionally compensating for drain-gate reverse leakage current effects; and

FIG. 9 is a graph illustrating the relationship between drain current and drain-source voltage in the biased transistor of the biasing circuit for a current mirror shown in FIG. 8.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A common solution to the problem of providing a controlled current in a biased device is to “mirror” a reference current (typically much smaller than the current in the biased device for DC current efficiency reasons) that flows in a reference device into the biased device. Typically, such circuits are known as “current mirrors”. Current mirrors generally utilize FET devices operating in the saturation region. The DC transfer characteristics of an FET device operation in the saturation region are described in the following equation:

I_(D)=(K/2)(W/L)[V_(GS)−(V_(to)−αV_(DS))]^(n)(1+λV_(DS)),  (Eq. 1)

where

I_(D)=drain current,

K=constant (depends on specific process parameters, e.g., layer thickness, carrier mobility, doping levels, etc.),

w=channel width,

L=channel length,

V_(GS)=gate-source voltage,

V_(to)=threshold voltage without threshold voltage modulation (V_(to)>0 for enhancement mode transistors; V_(to)<0 for depletion mode transistors),

V_(DS)=drain-source voltage,

α=threshold voltage modulation coefficient,

n=velocity saturation index (n=2 in a long channel device without velocity saturation; n=1 in an extreme short channel device with velocity saturation); in a typical 0.5 μm hannel length device n≈1.5, and

λ=channel length modulation coefficient (or “Early-Voltage” coefficient).

FIG. 1 illustrates a typical prior art current mirror, shown generally at 10, in silicon MOSFET technology using enhancement mode n-channel transistors Q₁ and Q₂ operating in the saturation region, each transistor Q₁,Q₂ having drain D, source S and gate G contacts. In operation, the gate currents I_(G1) and I_(G2) flowing into the gates G₁ and G₂ of transistors Q₁ and Q₂ are zero (or very small compared to the reference current I_(REF) supplied by current source 12). The drain-source voltage V_(DSI) of transistor Q₁ is equal to its gate-source voltage V_(GSI) due to the feedback loop 14. The feedback loop 14 adjusts the gate-source voltage V_(GS1) of transistor Q₁ such that the entire reference current I_(REF) will flow as the drain current of the “reference device” Q₁.

Since the gates G₁,G₂ of the two transistors Q₁,Q₂ are connected together with their sources S₁,S₂ connected to ground, and since there is no gate current flowing that would establish any voltage drop (I_(GI)=I_(G2)=0), the gate voltages applied to transistors Q₁ and Q₂ will be equal (V_(GS1)=V_(GS2)). Thus, the output current I_(OUT) will mirror the reference current I_(REF). The accuracy of the current mirror 10 is limited by the threshold voltage mismatch of the two transistors Q₁ and Q₂, as well as by short channel effects.

In order to achieve current multiplication ratios other than unity, the channel widths W₁,W₂ of the transistors Q₁,Q₂ must be different. In an ideal case, the channel width W₂ of Q₂ is an integer multiple of the channel width W₁ of Q₁. This will equalize the influence of short channel effects on the current multiplication ratio. In this case, I_(OUT) is related to the channel widths W₁,W₂ as follows:

I_(OUT)=I_(REF)(W₂/W₁).  (Eq. 2)

FIG. 2 illustrates a conventional current mirror, shown generally at 20, in GaAs MESFET technology using depletion mode n-channel transistors Q₁ and Q₂. The operation of the depletion mode current mirror 20 is identical to the operation of the enhancement mode current mirror 10 shown in FIG. 1, with the exception that measures have to be taken to account for the negative threshold voltage (V_(t)<0) of transistors Q₁,Q₂ in the depletion mode current mirror 20.

In order to ensure transistor operation in the saturation region, an additional level shift is necessary between the gate G₁ and drain D₁ of the reference transistor Q₁. This additional level shift is accomplished by a level shifter circuit 22 connected between the gate G₁ and drain D₁ of transistor Q₁ The level shifter 22 generally includes a series of diodes D₁ . . . D_(M) that are forward biased by a “helper current” I_(H) and voltage source V_(ss) (V_(ss)may be any negative voltage), and connected between the drain D₁ and gate G₁ of Q₁ through a “helper source follower” transistor Q_(H). The voltage gain of the level shifter 22 is unity and its effect on the current mirror 20 is similar to that of the feedback loop 14 as described with respect to FIG. 1.

FIG. 3 illustrates a conventional biasing circuit, shown generally at 30, utilized in common source amplifiers. A solution to the problem of biasing common source amplifiers is an extension of the current mirrors 10,20 shown in FIGS. 1 and 2 by the addition of two resistors R₁ and R₂ serially connected between the gates G₁ and G₂ of transistors Q₁ and Q₂, and a bypass capacitor C_(B) connected between a node 32 common to resistors R₁ and R₂ and ground. In the biasing circuit 30 shown in FIG. 3, depending upon the implementation with either enhancement mode (FIG. 1) or depletion mode (FIG. 2) transistors, the unity gain amplifier 34 may be replaced with either the level shifter 22 (with current source I_(H) and voltage source V_(SS)) of FIG. 2 or the feedback loop 14 of FIG. 1.

The bypass capacitor C_(B) accomplishes a low impedance at the center node 32 such that the resistor R₂, together with the gate input impedance of transistor Q₂, determine the overall input impedance seen by the signal SOURCE connected to the gate G₂ of transistor Q₂ through capacitor C_(K). Resistor R₁ should have a resistance equal to R₂×(W₂/W₁) to aid in reducing the effects of gate leakage current. In the case of a linear amplifier, such as a low noise amplifier or a linear power amplifier, the reference current I_(REF) should be chosen such that the gain of the biasing circuit 30 is independent of device tolerances. In the case of a saturated amplifier, the reference current I_(REF) should be chosen to be constant over temperature variations. Thus, assuming a unity gain amplifier 34, I_(REF)=I_(OUT).

One disadvantage in the circuits previously described, is that in the presence of short channel effects, the current multiplication ratio will be different from the geometrical value (W₂/W₁) in the case where the drain-source voltages V_(DS1) and V_(DS2) on the reference device Q₁ and the biased device Q₂ are different. Generally, it can be found that in MOSFET and MESFET devices, the channel length modulation coefficient λ increases as the channel length of the device decreases. In a common source amplifier case (FIG. 3) the minimum channel length available in the transistor technology in which the circuit is implemented is the most desirable one to use since it allows for the highest frequency of operation of the circuit. Thus, in order to achieve a desired drain current I_(D2) (I_(OuT)) operating point for the biased device Q₂ over drain-source voltage V_(DS2) variations, caused by, for example, supply voltage (V_(BB)) variations, the influence of λ must be compensated for.

Another disadvantage in the previously described circuits is that in the presence of threshold voltage modulation in short channel devices, the drain current I_(D2) (I_(OUT)) in the biased device Q₂ will again be affected by changes in the drain-source voltage V_(DS2).

FIG. 4 illustrates the drain current I_(D2) (I_(OUT)) in the biased device Q₂ as a function of its drain-source voltage V_(DS2) due to short channel and threshold voltage modulation effects. The transistors Q₁ and Q₂ utilized to generate the graph of FIG. 4 are PHEMT transistors having a channel length of 0.5 Am. The geometric channel width ratio (W₂/W₁) is unity, the reference current I_(REF) is 1 mA, and therefore the desired output current I_(D2) (I_(OUT)) is 1 mA. However, as shown in FIG. 4, the output current I_(D2) (I_(OUT)) increases as V_(DS2) increases. The deviation shown in FIG. 4 is entirely due to short channel and threshold voltage modulation effects.

Another disadvantage to the general solution, as shown in FIG. 3, for biasing common source amplifiers is that any gate leakage current I_(GL1), I_(GL2) out of the gates G₁,G₂ of transistors Q₁ and/or Q₂ will cause voltage drops across the serially connected resistors R₁ and R₂. Even if the two resistors R₁,R₂ are ratioed according to the geometrical channel width ratio such that W₂/W₁=R₂/R₁, the voltage drops across the resistors R₁ and R₂ will be different since the drain-source voltages V_(DS1) and V_(DS2) (and also the drain-gate voltages V_(DG1) and V_(DG2)) are not the same. Thus, the gate leakage currents I_(GL1) and I_(GL2) of transistors Q₁ and Q₂ will be different since the gate leakage current I_(GL1),I_(GL2) depends exponentially on the drain-gate voltages V_(DG1),V_(DG2) applied.

FIG. 5 illustrates the drain current I_(D2) (I_(OUT)) in the biased device Q₂ as a function of its drain-source voltage V_(DS2) due to the effects of gate leakage current. The circuit utilized to generate the graph of FIG. 5 follows the schematic shown in FIG. 3, with transistors Q₁ and Q₂ being PHEMT transistors having a channel length of 0.5 μm. The value of resistor R₂ is 850 Ω, the geometrical channel width ratio (W₂/W₁) is 75, and the desired output current I_(D2) (I_(OUT)) is 150 mA. The deviation of the measured current I_(D2) (I_(OUT)) from the desired value is due to the gate leakage current I_(GL2) of Q₂ causing a voltage drop across R₂ that is different from the voltage drop across the ratioed resistor R₁, which is caused by the gate leakage current I_(GL1) of Q₁.

FIG. 6 illustrates a current biasing circuit, shown generally at 40, according to the present invention for minimizing the effects of short channel lengths and threshold voltage modulation generally present in current mirror circuits. The current biasing circuit 40 includes a reference transistor Q₃ and a biased transistor Q₄, each having drain D, source S and gate G contacts. Resistors R₁₁ and R₁₂ are serially connected between the gates G₃ and G₄ of transistors Q₃ and Q₄, with a unity gain amplifier 42, or feedback loop, connected between a node 44 common to resistors R₁₁ and R₁₂ and the drain D₃ of Q₃. Depending upon the implementation of the unity gain amplifier 42 as either the level shifter 22 (with current source I_(H) and voltage source V_(SS) ) of FIG. 2, or the feedback loop 14 of FIG. 1, the biasing circuit 40 shown in FIG. 6 can be implemented as a current mirror in MOSFET and/or MESFET technologies. The addition of capacitor C_(B) makes it possible for the current biasing circuit 40 to be utilized in common source amplifiers (the signal SOURCE would be input to the gate G₄ of transistor Q₄).

The current biasing circuit 40 includes a compensation network 46 connected between transistors Q₃ and Q₄. The compensation network 46 includes a resistor R₂₁ connected between the gate G₃ of transistor Q₃ and the drain D₄ of transistor Q₄, and a resistor R₂₂ connected between the drain D₃ of the transistor Q₃ and the gate G₄ of transistor Q₄.

The sources S₃,S₄ of transistors Q₃,Q₄ are connected to ground. The gate currents IG₃, IG₄ are zero (or negligible with respect to I_(REF)), and accordingly, there is no voltage drop across resistors R₁₁ and R₁₂. Similarly, the currents through resistors R₂₁ and R₂₂ are negligible with respect to I_(REF). Since the drain D₃ and gate G₃ of transistor Q₃ are connected together, via unity gain amplifier 42, the bias or output current I_(OUT) (I_(D4)) mirrors a reference current I_(REF) which flows into the drain D₃ of Q₃ and is supplied by a current source 48. However, as previously noted, various operational parameters, such as short channel effects, threshold voltage modulation and gate leakage currents, influence the current multiplication factor and thus the output current I^(OUT) (I_(D4)). These operational parameters may result from a changing drain-source voltage V_(DS4) in transistor Q₄, resulting from variations in the battery voltage V_(BB) connected to the drain D₄ of transistor Q₄. The current biasing circuit 40 of FIG. 6 is designed to minimize the effects of these operational parameters.

For simplicity, it is assumed that there is only a threshold voltage (V_(t)) modulation effect influencing the transfer function of Eq. 1 (λ=0):

I_(D)=(K/2) (W/L) [V_(GS)−V_(t)]^(n),  (Eq. 3)

where

V_(t)=V_(to)−αV_(DS).  (Eq. 4)

The effective threshold voltage V₃ of transistor Q₃ is V_(to)−αV_(DS3), and the effective threshold voltage V_(t4) of transistor Q₄ is V_(to)−αV_(DS4). As the drain-source voltages V_(DS3),V_(DS4) of transistors Q₃,Q₄ change, so does their respective threshold voltage V_(t3),V_(t4). As the threshold voltages V_(t3),V_(t4) of transistors Q₃,Q₄ change, so does their respective drain currents I_(D3),I_(D4). From Eqs. 3-4, it follows that the difference between the two effective threshold voltages Vt₃ and V_(t4) is

(V_(t3)−V_(t4))=α(V_(DS4)−V_(DS3)).  (Eq. 5)

Since K, W and L in Eq. 3 are constants, the only way to compensate for a changing threshold voltage V_(t) (due to threshold voltage modulation effects, i.e., changing V_(DS)) is to modify V_(GS) such that V_(GS)−V_(t), where V_(t)=V_(to)−αV_(DS) (Eq. 4), remains constant regardless of changes in the drain-source voltages. This is accomplished by the compensation network 46 of FIG. 6 as follows.

The output of the unity gain amplifier 42 forces a voltage V_(CC) on its output at node 44. Basic circuit analysis reveals that the voltage on the gate G₃ of Q₃ (V_(GS3)) is higher than V_(CC) by the amount (V_(DS4)−V_(CC)) [R₁₁/(R₁₁+R₂₁)], and similarly, the voltage on the gate G₄ of Q₄ (V_(GS4)) is higher than V_(CC) by the amount (V_(DS3)−V_(CC)) [R₁₂/(R₁₂+R₂₂)].

For symmetry reasons in a unity current gain mirror, R₁₁ =R₁₁=R₁, and similarly R₂₁=R₂₂=R₂. Accordingly, after simple algebraic manipulation, the difference of the two gates voltages V_(GS3),V_(GS4) is

(V_(GS3)−V_(GS4))=[R₁/(R₁+R₂)](V_(DS4)−V_(DS3)).  (Eq. 6)

Comparing Eq. 5 and Eq. 6, the difference of the gate-source voltages (V_(GS3)−V_(GS4)) of transistors Q₃ and Q₄ can be made equal to the difference of their effective threshold voltages (V_(t3)−V_(t4)) if the following design choice is made: α=R₁/(R₁+R₂).

This is the appropriate design choice for cancellation of the threshold voltage modulation effects, and thus the influence of changing drain-source voltages V_(DS3),V_(DS4), on the output current I_(OUT) (I_(D4)).

In the presence of short channel effects, the parameter λ in the transfer function of Eq. 1 has a non-zero value and must be taken into account. The effect of λ is similar to the effect of α, in that λ models the dependence of the drain current I_(D) in transistors operating in the saturation region on their drain-source voltage V_(DS). This dependence stems from channel length modulation, L→(L−ΔL), with ΔL increasing with increasing V_(DS). This leads to an additional factor in the drain current I_(D) equation: I_(D)→I_(D)×(1+λV_(DS)).

Adding this additional factor to the transfer function of Eq. 1, the drain currents I_(D3),I_(D4) for the transistors Q₃ and Q₄ in FIG. 6 are:

I_(D3)=(K/2) (W/L) [V_(GS3)−(V_(to)−αV_(DS3))]^(n)(1+λV_(DS3)),  (Eq. 7)

I_(D4)=(K/2) (W/L) [V_(GS4)−(V_(to)−αV_(DS4))]_(n)(1+λV_(DS4)).  (Eq. 8)

For compensation effects, it is assumed that the current through resistors R₁₁ and R₁₂ (R₁) and R₁₂ and R₂₂ (R₂) is negligible with respect to I_(REF). Thus, I_(D3) is approximately equal to I_(REF).

Assuming a 1:1 current mirror, if the drain current I_(D4) (I_(OUT)) through transistor Q₄ is to remain constant regardless of changes in V_(DS3) and/or V_(DS4), then it follows that: $\begin{matrix} {\frac{\partial I_{D4}}{\partial V_{DS4}} = {\frac{\partial I_{D4}}{\partial V_{DS3}} = 0.}} & \text{(Eq. 9)} \end{matrix}$

Basic circuit analysis of the current biasing circuit 40 of FIG. 6 reveals that

V_(GS3)=[R₂/(R₁+R₂)]V_(CC)+[R₁/(R₁+R₂)]V_(DS4), and  (Eq. 10)

V_(GS4)=[R₂/(R₁+R₂)]V_(CC)+[R₁/(R₁+R₂)]V_(DS3).  (Eq. 11)

After algebraic elimination of V_(CC),

V_(GS4)=V_(GS3)+[R₁/(R₁+R₂)](V_(DS3)−V_(DS4)).  (Eq. 12)

By virtue of the unity gain amplifier 42, and the fact that I_(D3)=I_(REF),

V_(GS3)=(V_(to)−αV_(DS3))[I_(REF)/((1+λV_(DS3))(KW/2L))]^(1/n)  (Eq. 13)

Eq. 12 and Eq. 13 yield expressions that can be used to evaluate the partial derivatives of I_(D4) (Eq. 8) with respect to V_(DS3) and V_(DS4) (Eq. 9). After calculation of the partial derivatives, a modified value for the appropriate values of the resistors is obtained, namely,

R₁/(R₁+R₂)=α+(λ/n) (V_(GS3)−V_(t3)).  (Eq. 14)

Since Vt₃ will be provided by the manufacturer of the transistor device Q₃, and V_(GS3) can be determined by knowledge of I_(REF) (I_(D3)), resistors R₁ (R₁₁ and R₁₂) and R₂ (R₂₁ and R₂₂) can be chosen to obtain the appropriate ratio of Eq. 14. This is the appropriate design choice for cancellation of threshold voltage modulation and short channel effects on the output current I_(OUT) (I_(D4))

FIG. 7 illustrates the drain current I_(D4) (I_(OUT)) of Q₄ as a function of its drain-source voltage V_(DS4) for the circuit of FIG. 6. Transistors Q₃ and Q₄ are PHEMT transistors each having a channel length of 0.5 μm. The geometric channel width ratio (W₂/W₁) is unity, with values for resistors R₁₁, R₁₂, R₂₁ and R₂₂ chosen as follows: R₁₁=1 kΩ; R₁₂=1 kΩ; R₂₁=50 kΩ; and R₂₂=50 kΩ. As seen from FIG. 7, the drain current I_(D4) (I_(OUT)) through Q₄ remains constant regardless of changes in its drain-source voltage V_(DS4). Since a unity gain amplifier was assumed, the drain current I_(D4) (I_(OUT)) equals the reference current I_(REF), which is approximately 1 mA.

FIG. 8 illustrates a biasing circuit according to a second embodiment of the present invention, shown generally at 50, with like elements of FIG. 6 indicated with the same reference numbers and elements that have been modified indicated with a prime (′). In this second embodiment, the compensation network 46′ further includes an additional compensation network 52 including transistor Q₅ and resistor R₄. Devices Q₅ and R₄ are added to minimize the effects of drain-gate reverse leakage currents as previously described. The drain D₅ of transistor Q₅ is connected to the drain D₄ of transistor Q₄, with the gate G₅ of transistor Q₅ connected to the gate G₃ of transistor Q₃. The resistor R₄ is connected between the source S₅ of transistor Q₅ and ground. The biasing circuit 50 is of particularly utility for large current multiplication ratios. The reason being that the absolute magnitudes of the drain-gate reverse leakage currents I_(GL3) and I_(GL4) of transistors Q₃ and Q₄ differ more for larger multiplication ratios. While this difference could be offset by ratioing the resistor values R₁₁/R₁₂ and R₁₂/R₂₂ according to the current mirror ratio, for large ratios this leads to unreasonably high resistance values for R₁₁ and R₁₂. In addition, this approach does not work for a wide range of drain-source voltages V_(DS) of the biased transistor Q₄, but is only valid if the drain-source voltages V_(DS3) and V_(DS4) of both transistors Q₃ and Q₄ are equal.

Since large resistors generally consume a large amount of chip space and are not economical for monolithic integration, the total amount of chip area consumed by transistor Q₅ and resistor R₄, can be reduced by the addition of resistors R₃₁ and R₃₂. Resistor R₃₁ is connected between resistors R₁₁-R₂₁ and the gate G₃ of transistor Q₃, while resistor R₃₂ is connected between resistors R₁₂-R₂₂ and the gate G₄ of transistor Q₄.

The addition of resistors R₃₁ and R₃₂ permits scaling of resistors R₁₂ and R₂₂ by a scaling factor S₂<1, e.g., R₁₂=S₂R₁₂ and R₂₂ 32 S₂R₂₂, with resistor R₃₂ chosen to be R₃₂=R₂₂(1−S₂). The scaling factor S₂ should be made as small as possible in a practical design, but big enough to keep the current I_(D5) flowing in the compensation network 52 (Q₅ and R₄) below 5% to 10% of the reference current I_(REF). It should be noted that the compensation network 52 (Q₅ and R₄) can be equally applied to both sides of the current mirror.

Operation of the biasing circuit 50 of FIG. 8 in minimizing drain-gate current leakage is as follows. Assume a large desired current multiplication factor, e.g., 75 as in a typical power amplifier application. Since Q₄ will be sized much larger than Q₃ (75×in the present example), the leakage current I_(GL3) of the reference transistor Q₃ can be neglected with respect to the leakage current I_(GL4) in the biased transistor Q₄. As a practical matter, the gate leakage currents for each transistor are known a priori, as the manufacturer of the device provides this information on the transistor spec sheet.

The transistor Q₅ is chosen such that its channel length is the same as the other transistors Q₃ and Q₄ in the biasing circuit 50. As previously discussed, the gate-source voltage V_(GS3) of transistor Q₃ is ideally the same as the gate-source voltage V_(GS4) of transistor Q₄ (I_(G3)=I_(G4)=0). It follows then, that the drain-gate voltage V_(DG5) of transistor Q₅ is equal to the drain-gate voltage V_(DG4) of the biased transistor Q₄. This results in the same gate leakage current densities (gate leakage current per area) in both devices Q₄ and Q₅. From the area ratios of transistors Q₄ and Q₅, the actual gate leakage current I_(GL5) flowing out of the gate G₅ of transistor Q₅ can be determined.

The leakage current I_(GL5) flowing out of the gate G₅ of transistor Q₅ creates a voltage drop V_(GL5) across the resistor series connection R₃₁ and R₁₁. Similarly, the leakage current I_(GL4) flowing out of the gate G₄ of transistor Q₄ creates a voltage drop V_(GL4) across the resistor series connection R₃₂ and R₁₂. Resistors R₃₁, R₁₁, R₃₂ and R₁₂ are chosen such that V_(GL5)=V_(GL4).

Due to the action of the feedback loop (amplifier 42) around the reference device Q₃, the gate voltage V_(GS3) of transistor Q₃ is held constant and the voltage at the output (node 44) of the unity gain amplifier 42 is lowered. This will then lower the gate voltage V_(GS4) of transistor Q₄ and thereby reduce the drain current I_(D4) (I_(OUT)) of transistor Q₄.

The drain current I_(D5) through transistor Q₅ is limited to a small value by resistor R₄ which forces the gate-source voltage V_(GS5) of transistor Q₅ to be close to the gate-source voltages V_(GS3) and V_(GS4) of transistors Q₃ and Q₄. In this manner, it is ensured that the reverse gate leakage current densities are equal for transistors Q₄ and Q₅. The amount of drain current I_(D5) in transistor Q₅ does not influence the accuracy of the compensation network 52 (Q₅ and R₄), however, it should be kept small.

In an ideal case, the channel widths W₃,W₅ of the transistors Q₃ and Q₅ are integer multiples of each other with the channel width W₅ of Q₅ smaller than the channel width W₄ of Q₄ (W₄=NW₅, with N>>1). Although this is not a requirement for proper operation of the biasing circuit 50, the chip area consumption due to the addition of the gate leakage compensation network 52 (Q₅ and R₄) is kept at a minimum.

Resistors R₁₁ and R₂₁ can be scaled by a scaling factor S₁ using the requirement that the current through the series connection of S₁R₂₁−S₁R₁₁ should be the same as the current through the series connection of S₂R₂₂−S₂R₁₂. This balances the current sum at the output of the unit gain amplifier 42 at node 44.

The scaling factor S₁ is chosen to be S₁=S₂(V_(DS4)/V_(DS3)). This leads to values for the resistors as follows: R₁₁=S₁R₁₁ and R₂₁=S₁R₂₁. To balance the gate-source voltage shifts on both sides of the current mirror, and thus ensure a constant drain current I_(D4) (I_(OUT)), the resistor R₃₁ should be scaled accordingly:

R₃₁=R₃₁[(W₄/W₃)/(N+1)−S₁]/(1−S₂).

FIG. 9 illustrates the drain current I_(D4) (I_(OUT)) of transistor Q₄ as a function of its drain-source voltage V_(DS4) achieved by the biasing circuit 50 of FIG. 8. Transistors Q₃, Q₄ and Q₅ are PHEMT transistors each having a channel length of 0.5 cm. The geometric channel width ratio (W₄/W₃) is 75. The reference current I_(REF) is 2 mA. The resistor values are as follows: R₁₁=680 Ω; R₁₂=255 Ω; R₂₁=31 kΩ; R₂₂=11.6 kΩ; R₃₁=6.8 kΩ; R₃₂=595 Ω; and R₄=10 kΩ. As illustrated, the desired output current I_(OUT) (I_(D4)) is 150 mA (equal to 75×the reference current I_(REF)) over a changing drain-source voltage V_(DS4) in the biased device Q₄.

While the invention has been described with particular reference to the drawings, it should be understood that various modifications could be made without departing from the spirit and scope of the present invention. 

What is claimed is:
 1. A current mirror circuit comprising: a reference device having control, input and output elements; a reference current source connected to the input element ofthe reference device, said reference source producing a reference current flowing through the reference device; a biased device having control, input and output elements, the control element of the biased device operably connected to the control element of the reference device, wherein a bias current is produced in the biased device as a multiple of the reference current; and a compensation network connected between the biased device and the reference device, the compensation network comprising a resistor network maintaining the bias current constant regardless of varying voltage across at least one ofthe biased device and the reference device.
 2. The current mirror circuit of claim 1, wherein the reference and biased devices comprise field effect transistors having gate, drain and source elements corresponding to the control, input and output elements.
 3. The current mirror circuit of claim 2, wherein the reference current flows from the drain to source elements in the reference transistor, the biased current flows from the drain to source elements in the biased transistor, and the varying voltage comprise a varying voltage across the drain and source elements of at least one of the biased transistor and the reference transistor.
 4. The current mirror circuit of claim 3, wherein the varying voltage across the drain and source elements of at least one of the biased transistor and the reference transistor results from at least one of threshold voltage modulation, short channel effects and gate leakage current occurring in at least one of the biased transistor and the reference transistor.
 5. A current mirror circuit comprising: a reference device having control, input and output elements; a reference current source connected to the input element of the reference device, said reference current source producing a reference current flowing through the reference device; a biased device having control, input and output elements, the control element of the biased device operably connected to the control element of the reference device, wherein a bias current is produced in the biased device as a multiple of the reference current; and a compensation network connected between the biased device and the reference device, the compensation network maintaining the bias current constant regardless of varying operating characteristics in at least one of the biased device and the reference device, the compensation network comprising a first resistor connected between the input element of the reference device and the control element of the biased device, and a second resistor connected between the input element of the biased device and the control element of the reference device.
 6. The current mirror circuit of claim 5, further comprising: a third resistor connected between a first node and the control element of the reference device; a fourth resistor connected between the first node and the control element of the biased device; and a feedback loop provided between the first node and the input element of the reference device.
 7. The current mirror circuit of claim 6, wherein the feedback loop comprises a unity gain amplifier.
 8. The current mirror circuit of claim 6, wherein the feedback loop comprises a level shifter biasing the reference device to operate in a saturation mode.
 9. The current mirror circuit of claim 6, wherein the first and second resistors have equal resistance values, and wherein the third and fourth resistors have equal r esistance values.
 10. The current mirror circuit of claim 6, wherein the compensation network further comprises: a compensation device having control, input and output elements, the input element of the compensation device connected to the input element of the biased device, and the control element of the compensation device connected to the control element of the reference device; and a fifth resistor connected between the output element of the compensation device and ground.
 11. The current mirror circuit of claim 10, wherein the compensation device comprises a field effect transistor having gate, drain and source elements corresponding to the control, input and output elements.
 12. The current mirror circuit of claim 10, wherein the compensation network further comprises: a sixth resistor connecting the second and third resistors to the control element of the reference device; and a seventh resistor connecting the first and fourth resistors to the control element of the biased device.
 13. A current mirror circuit comprising: a reference transistor having control, input and output elements; a reference current source connected to the input element of the reference transistor, the reference current source producing a reference current flowing through the reference transistor; a biased transistor having control, input and output elements, the control element of the biased transistor operably connected to the control element of the reference transistor, wherein a bias current is produced in the biased transistor as a multiple of the reference current; first and second resistors serially connected between the control elements of the reference and biased transistors; and a compensation network connected between the biased transistor and the reference transistor for maintaining the bias current constant regardless of varying operating characteristics in at least one ofthe biased transistor and the reference transistor, said compensation network comprising: a third resistor connected between the input element ofthe reference transistor and the control element of the biased transistor; and a fourth resistor connected between the input element of the biased transistor and the control element of the reference transistor.
 14. The current mirror circuit of claim 13, further comprising a unity gain amplifier connected between the input element of the reference transistor and a node common to the first and second resistors.
 15. The current mirror circuit of claim 13, wherein the reference and biased transistors comprise field effect transistors having gate, drain and source elements corresponding to the control, input and output elements, respectively.
 16. The current mirror circuit of claim 15, wherein the reference and biased transistors comprise metal oxide semiconductor field effect transistors.
 17. The current mirror circuit of claim 15, wherein the reference and biased transistors comprise metal semiconductor field effect transistors.
 18. The current mirror circuit of claim 13, further comprising a bypass circuit connected between the node common to the first and second resistors and ground.
 19. The current mirror circuit of claim 18, wherein the bypass circuit comprises a capacitor.
 20. The current mirror circuit of claim 13, wherein the output elements of the reference and biased transistors are connected to ground.
 21. The current mirror circuit of claim 13, wherein the first and second resistors have equal resistance values.
 22. The current mirror circuit of claim 13, wherein the third and fourth resistors have equal resistance values.
 23. The current mirror circuit of claim 13, wherein the compensation network further comprises a compensation circuit connected between the reference and biased transistors compensating for leakage current in the reference and biased transistors.
 24. The current mirror circuit of claim 23, wherein the compensation circuit comprises a compensation transistor having control, input and output elements, the input element of the compensation transistor connected to the input element of the biased transistor, and the control element of the compensation transistor connected to the control element of the reference transistor.
 25. The current mirror circuit of claim 24, wherein the compensation circuit further comprises a fifth resistor connected between the output element of the compensation transistor and ground.
 26. The current mirror circuit of claim 24, wherein the compensation transistor comprises a field effect transistor having gate, drain and source elements corresponding to said control, input and output elements.
 27. The current mirror circuit of claim 26, wherein the compensation transistor comprises a metal oxide semiconductor field effect transistor.
 28. The current mirror circuit of claim 26, wherein the compensation transistor comprises a metal semiconductor field effect transistor.
 29. The current mirror circuit of claim 23, wherein the compensation network further comprises: a sixth resistor connecting the first and fourth resistors to the control element of the reference transistor; and a seventh resistor connecting the second and third resistors to the control element of the biased transistor. 